When the plastic melts, it cements rock fragments, sand, and shell debris together, or the plastic can flow into larger rocks and fill in cracks and bubbles to form a kind of junkyard Frankenstein.

[Patricia] Corcoran says some of the plastic is still recognizable as toothbrushes, forks, ropes, and just "anything you can think of." Once the plastic has fused to denser materials, like rock and coral, it sinks to the sea floor, and the chances it will become buried and preserved in the geologic record increase.

Corcoran and her team canvassed Kamilo Beach on the Big Island for more of the rocks and found plastiglomerate in all 21 sites they surveyed. She says people have already found plastiglomerate on another Hawaiian island, and she expects there to be much more on coastlines across the world. Plastiglomerate is likely well distributed, it's just never been noticed before now, she says.

The researchers don't believe the plastiglomerate will last forever, adding that the plastics might actually "revert back to a source of oil from whence they came, given the right conditions of burial."